Aliphatic polycarbonate diols have been known for a long time. They can be prepared from non-vicinal diols by reaction with diaryl carbonates (DE-OS 19 15 908), dialkyl carbonates (DE-OS 25 55 805), dioxolanones (DE-OS 25 23 352), phosgene (DE-OS 15 95 446), bischloroformates (DE-OS 8 57 948) or urea (Angew. Chem. 92 (1980) 742). From among the many diols described for use in the literature, only 1,6-hexanediol or compounds derived from 1,6-hexanediol have been widely used on an industrial scale. Thus, for example, high-quality polyurethane elastomers and also lacquers are prepared using polycarbonatediols which are based on 1,6-hexanediol.
The resistance to hydrolysis of polyurethanes prepared from these types of polycarbonatediols is particularly outstanding. It exceeds that of analogous compounds made from polyadipatepolyols by far. Pure hexanediolpolycarbonates with number-average molecular weights of 500 to 5000 are waxy substances with a softening temperature range of approx. 45 to 55° C., depending on the molecular weight. Accordingly, the polyurethanes prepared from these have an elevated shear modulus at low temperatures, i.e. they lose their flexibility. For this reason, diols were developed which were intended to compensate for this disadvantage. The following may be mentioned, for example: oligoesters based on adipic acid (DE-AS 19 64 998), oligoesters based on caprolactone (DE-AS 17 70 245) or oligomeric tetraethylene glycols (DE-AS 22 21 751) and tetrabutylene glycols.
The disadvantage with these building blocks is their more readily hydrolyzable ester group and the elevated hydrophilicity, which at the least leads to a greater swelling of the PUR molded items prepared therefrom.
Another disadvantage of polycarbonatediols based on hexanediol is the comparatively high viscosity (approx; 5000 mPas at 60° C. with a number-average molecular weight of 2000 g/mol). This can lead to problems during processing to form polyurethane molded items.
According to the disclosure in U.S. Pat. No. 4,808,691, the disadvantages mentioned can be overcome by reacting oligomeric hexanediols with diphenyl carbonate in a stoichiometric ratio such that polycarbonatediols with number-average molecular weights of 500 to 12000, preferably 700 to 6000, result, wherein the degree of oligomerization of the ether is chosen in such a way that the ratio of ether to carbonate groups is 5:1 to 1:5, preferably 3:1 to 1:3.
However, on an industrial scale, the method for preparing oligomeric hexanediols disclosed in U.S. Pat. No. 4,808,691 proves time-consuming, labor-intensive and therefore costly. The method described in detail in that document requires the separation of water of reaction by distillation, wherein an entraining agent and catalysts are also used. Naphthaline-1,5-disulfonic acid is mentioned as the preferred catalyst. Toluene, xylene, gas oil fractions, cyclohexane, chlorobenzene and the oxepane also being formed as a secondary product are used as entraining agents, wherein the formation of this product can be slightly suppressed.
Working up after reaching the desired degree of oligomerization, detectable from the amount of water formed, is performed, according to U.S. Pat. No. 4,808,691, in such a way that the reaction mixture is cooled to 100° C. and the sulfonates present in the reaction mixture are hydrolyzed by adding 5–10% of water over 1–3 hours. The catalyst released is neutralized with aqueous alkali or ammonia, the water and other volatile components are distilled off and the catalyst, deposited as a salt, is filtered off.
On an industrial scale, however, this type of procedure has two serious disadvantages: the salt is produced in a form that can be isolated only in a very lengthy process. Typical filtration times for a batch of 3 tons are of the order of magnitude of 30 hours, wherein a change of filter is required every hour. Smaller batches of, for example, 200 kg require filtration times of approx. 5 hours.
The use of coarser filter materials which, although they facilitate faster filtration, lead to less effective separation of the salt, cannot be considered because even the smallest amount of this salt has a negative effect on the reaction of the polyols obtained to give polycarbonatediols.
Furthermore, it has been shown that oligomeric hexanediols prepared in accordance with U.S. Pat. No. 4,808,691 cannot then be used universally. Although they can react with diphenyl carbonate to give the corresponding polycarbonatediols under catalysis with bis(tributyltin) oxide or dibutyltin oxide, they do not react using heavy-metal-free catalysis, e.g. using basic salts of magnesium such as magnesium hydroxide carbonate. Even the slightest contamination with salts of sulfonic acid leads to incomplete building up of the polycarbonatediol.
The use of organotin-containing catalysts is therefore a particular disadvantage because they are not ecologically harmless.
Furthermore, oligomeric hexanediols prepared in accordance with U.S. Pat. No. 4,808,691 exhibit comparatively strong discoloration that is then carried through to the corresponding secondary-products, that is polyethercarbonatepolyols, their diisocyanate prepolymers and also the polyurethane molded items produced therefrom. This undesired discoloration represents yet another disadvantage.
Another disadvantage is that hydrolysis exclusively with water and ammonia can lead to incomplete hydrolysis of the sulfonates. If these esters decompose during the course of further reactions, then the sulfonic acid released can also cause incomplete reactions.